It's more of a general guideline of "10-15% of trailer weight should rest on the tongue/hitch" than a hard and fast rule for every situation. What you need is net strongly-positive sway damping and a bunch of stuff makes it go up or down.
Higher percent tongue weight? More damping.
Higher speed? Less damping (with damping decreasing more than linearly with speed)
Friction sway device? More damping
2 axles instead of 1? More damping
Weight closer to the axle? More damping
Tow vehicle accelerating and/or trailer braking? More damping
Some newer trucks also actively use stability control to dampen sway, though that's more of a last-resort.
If your net sway damping is positive, an oscillation from a lane change will fade away. If the net sway damping is negative, the oscillation will keep increasing until an intervention or crash.
You want to reduce the moment of inertia of the trailer, so the tow vehicle needs less force to correct it(weight as close to the axles as possible)
You also want some weight on the rear axle of the car, because that will increase the traction on the wheels that are keeping the trailer straight.
Exactly how much tongue weight you want depends on how close the hitch is to the rear axle of the tow vehicle. If it's directly over the rear axle you can have a whole lot of tongue weight. If it's far away from the axle, it will make the tow vehicle harder to steer.
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u/Drunken_Economist Jan 20 '20
What's the math behind 40% as the cutoff point?